In vitro drug release rates from aqueous and oil solutions as well as preformed and in situ formed aqueous and oil suspensions intended for intra-articular delivery have been investigated using the rotating dialysis cell model. Using lidocaine as a model drug substance the release kinetics from aqueous and oil suspensions have been compared and the sustained release properties from such suspensions formed in situ has been evaluated. The appearance of lidocaine in the acceptor phase after instillation of preformed and in situ formed aqueous and oil suspensions into the small aqueous donor compartment applied to zero-order kinetics as long sufficient amounts of solid lidocaine remained in the donor cell. The obtained data indicate that oil solutions and oil suspensions of lidocaine possess prolonged release properties equal to or better than those of aqueous counterparts. Also the release properties of preformed aqueous and oil suspensions are identical to such suspension types formed in situ. The present in vitro model appears useful in quality control and formulation development in the field of parenteral depots.